open-pr: actually use the cached-and-restored git-sdk-64 subset

We can now successfully restore the cached artifact, but we still have
to add it to the `PATH` and set `MSYSTEM` accordingly...

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/open-pr.yml

@@ -118,6 +118,10 @@ jobs:
 
           # makepkg/updpkgsums expects `curl` to be present in `/usr/bin/`
           printf '#!/bin/sh\n\nexec /mingw64/bin/curl.exe "$@"' >usr/bin/curl &&
+      - name: use git-sdk-64 subset
+        shell: bash
+        run: |
+          cd .sdk &&
 
           # add the SDK directories to the `PATH`
           cygpath -aw "usr/bin/core_perl" >>$GITHUB_PATH &&